[mirimir]

Approved drugs are listed here: https://www.accessdata.fda.gov/scripts/cder/daf/

Also: https://www.accessdata.fda.gov/scripts/cder/ob/index.cfm

And: https://www.drugs.com/history/

Physicians can only prescribe FDA-approved drugs. Only brand drugs are approved on their own. Generic drugs are approved, with much less documentation required. There's no need to demonstrate effectiveness, safety, etc. Applicants must only demonstrate that products are therapeutically equivalent to approved brand drugs. That's mainly about chemical identity, purity, and rate and degree of absorption.

At Drugs@FDA, search for a brand name. If there are generics, you'll see "Therapeutic Equivalents for NDA ...". Clicking that will display links to them.

If there's no listed brand drug, there's nothing for a generic to be equivalent to.

[Zircom]

That didn't answer the question of why companies are allowed to pull drugs from being listed in the first place, after them being listed for years.

[mirimir]

I'm not sure what the requirements for delisting products are. It might be as simple as "this is old, and we don't want to be associated with it". Or as specific as "we've learned that it's not as safe and/or effective as it should be, and should be delisted for health and safety reasons."

But the real reason is almost always that they have a new version, which costs much more, and they don't want competition from inexpensive generics to their old product.

There have been some antitrust suits over that. I don't know how they turned out.